The operation amplifiers are useful in many different analog applications. For example, operational amplifiers are commonly found and utilized in communication systems. One application that requires operation amplifiers is a power line communications system. In such systems, the typical resistive loading is about a few ohms.
In this environment, ordinary operational amplifiers are insufficient because prior art amplifiers do not provide sufficient drive current. Unfortunately, if a heavy resistive load is applied to this amplifier, the output stage current will be insufficient to drive the load unless the biasing current of the output stage is set at an unreasonable level. For example, prior art operational amplifiers that use a class A output is not suitable for driving the type of resistive load encountered in power line communication systems.
Furthermore, in the design of multi-stage operational amplifiers instability and signal distortion are always of concern to the designer. In power line communication systems, stability issues are of even greater concern because the power line impedance changes with time and can become very low sometimes. Prior art operation amplifiers often exhibit an undesirable AC response, which can cause the amplifier to become unstable during operation. Large signal distortion is undesirable because the distortion prohibits the effective and efficient transmission of signals.
An example of an approach is described in U.S. Pat. No. 5,212,455, Pernici et al., “Differential Output, Power, CMOS, Operational Amplifier,” May 18, 1993. Unfortunately, the implementation has large distortions due to its unbalanced gains in the PMOS path and the NMOS path. For example, the total effective gain in the PMOS path is the product of the gains for the first stage, the second stage, and the third stage, whereas the total effective gain in the NMOS path is only the product of the gains of the first stage and the third stage.
Based on the foregoing, there remains a need for an operational amplifier that overcomes the disadvantages set forth previously.